Fluid in many producing wells is elevated to the surface by the action of a pumping unit or pumping apparatus installed in the lower portion of the well bore. In recent times there has been increased activity in the drilling of well bores to great depths. The use of water flooding as a means of secondary recovery of oil or other hydrocarbon fluids, after the production thereof has been somewhat depleted, is commonly practiced. Because water flooding produces a considerable quantity of fluid in the producing well bore it is preferable to provide a downhole pumping system capable of producing large quantities of fluid. Electrical submersible pump (ESP) systems have been found to meet this need. The electric motor that is typically used in such systems generates considerable heat. The motor is typically cooled by the transfer of heat to the surrounding annular fluids. In many cases, the pumping unit is set above perforations in the well casing so that the unit can make use of flowing well fluid to produce some convection cooling about the motor. Insufficient fluid velocity will cause the motor to overheat and may lead to early motor failure.
Fluid produced by the pumping unit consists of formation water, oil and quantities of gas. The presence of gas can be significant because gas inhibits the pump from producing liquid, which may result in gas blocking, or locking. Equipment failure may result if a unit is not shut down quickly after gas blocking. It is therefore desirable to place the pump below the well casing perforations to take advantage of the natural annular separation of the gas from the liquid. However, by placing the pump below casing perforations, the motor of the pumping unit is not exposed to flowing well fluid that normally provides cooling to the motor of the electrical submersible pump. As a result, a motor in a pumping unit placed below casing perforations tends to overheat and may experience a shortened operational life unless a means for circulating fluid over the surface of the motor is provided.
In some applications, fluid flow past the motor is achieved by drawing fluid through the annulus between the motor and the casing. Disadvantages associated with this arrangement include scale deposited by the fluid in proximity to the hot motor. The scaling problem is exacerbated by the pressure drop associated with drawing the fluid through the annular space surrounding the motor. Scale deposits can block fluid flow and may result in increased difficulties when attempting to remove the electrical submersible pump.